The present invention relates to a control system for a continuously variable belt-drive automatic transmission for a motor vehicle, and more particularly to a system for controlling line pressure in a hydraulic circuit for the transmission.
A known control system for a continuously variable belt-drive transmission disclosed in U.S. Pat. No. 4,369,675 comprises an endless belt running over a drive pulley and a driven pulley. Each pulley comprises a movable conical disc which is axially moved by a fluid operated servo device so as to vary the running diameter of the belt on the pulleys in dependency on driving conditions. The system is provided with a hydraulic circuit including a pump, a line pressure control valve and a transmission ratio control valve. Each valve comprises a spool to control the oil supplied to the servo devices.
The transmission ratio control valve operates to control the transmission ratio in accordance with the opening degree of a throttle valve of an engine and the speed of the engine. The line pressure control valve is adapted to control the line pressure in accordance with the transmission ratio and the engine torque. The line pressure is controlled to prevent the belt from slipping on the pulleys in order to transmit the output of the engine. However, the control operation for the line pressure is liable to be delayed in a transient state driving condition of the vehicle, where the engine torque and transmission ratio change rapidly.
Japanese Patent Laid Open No. 58-191361 discloses a control system for controlling the line pressure in the transient state driving condition of a vehicle. In the control system, the difference between an actual speed of an engine and a desired speed or the difference between an actual transmission ratio and a desired transmission ratio is detected. A rapid change in the transient state is detected by a larger difference than a predetermined value. At the rapid change, the line pressure is increased for a period. Although the system responds to the change of the transmission ratio without large delay, the system does not quickly respond to the change of engine torque.
Generally, the transmission ratio is determined by the opening degree of a throttle valve and speed of a vehicle. Since the vehicle speed does not quickly change, a rapid change of the transmission ratio can be preliminarily detected by a rapid change of the opening degree of the throttle valve. On the other hand, the engine torque is determined by engine speed and opening degree of the throttle valve. When the transmission ratio is constant, the engine speed is proportional to the vehicle speed and does not immediately change. Accordingly, a rapid change of engine torque can also be detected by a rapid change of the opening degree of the throttle valve.